The invention relates to an analyser for continuously measuring the H2S contained in a gas. It also relates to a device including the said analyser for regulating the flow rate of air injected into a reactor for oxidizing H2S to sulphur.
In order to recover the H2S present in low concentration, especially a concentration of less than 5% by volume, in gases of various origins, it is common practice to use processes involving oxidation, especially catalytic oxidation, of H2S to sulphur according to the reaction H2S+{fraction (1/2)} O2xe2x86x92S+H2O.
In such oxidation processes, the gas to be treated containing H2S in the presence of a controlled amount of a gas containing free oxygen is made to come into contact with a catalyst for selective oxidation of H2S to sulphur, the said contact being achieved at temperatures either above the dew point of the sulphur formed, in which case the sulphur formed is present in the vapor state in the gaseous effluent resulting from the reaction, or at temperatures below the dew point of the sulphur formed, in which case the said sulphur is deposited on the catalyst, thereby requiring the sulphur-laden catalyst to be periodically regenerated by purging, by means of a non-oxidizing gas having a temperature of between 200xc2x0 C. and 500xc2x0 C. The gas containing free oxygen used for oxidizing the H2S to sulphur is usually air, but it may also consist of oxygen, oxygen-enriched air or else mixtures, in various proportions, of oxygen and an inert gas other than nitrogen. In the following, xe2x80x9cairxe2x80x9d is used to denote the said gas containing free oxygen.
The amount of air, with which the gas to be treated containing H2S is combined, is continuously adjusted in response to a parameter resulting from the superposition of a prediction parameter, representative of an air flow rate corresponding to an amount of oxygen proportional to the amount of H2S present in the gas to be treated and injected into the oxidation reactor, and of a correction parameter (a feedback parameter), representative of a corrective air flow rate for bringing the H2S content present in the gaseous effluent coming from the oxidation back to a set value.
The oxidation is carried out in a reactor having an upstream end and a downstream end which are advantageously separated by a bed of a catalyst for selective oxidation of H2S to sulphur, the said upstream end being equipped with a first line and a second line for the injection of the gas to be treated and of air into the reactor, respectively, and the said downstream end being equipped with an output line for the gases, in order to discharge the gaseous effluent resulting from the oxidation, and the flow rate of air injected into the oxidation reactor is adjusted with the aid of a regulating device combining (i) a prediction unit, which comprises a prediction computer receiving a signal from a flow meter and a signal delivered by a first H2S-content analyser, these being mounted in the first line at the upstream end of the oxidation reactor and generating, from the said signals, a signal representative of an air flow rate corresponding to an amount of oxygen proportional to the H2S content entering the oxidation reactor with (ii) a feedback unit, which comprises a correction computer receiving a signal delivered by a second H2S-content analyser, mounted in the output line of the oxidation reactor and generating, from the said signal, a signal representative of a corrective air flow rate in order to bring the H2S content present in the gaseous effluent passing through the said output line back to a given set value and with (iii) a flow regulator, which receives the signals generated by the prediction and correction computers and the signal delivered by a flow meter, mounted in the air injection line at the upstream end of the oxidation reactor and applying, to a valve with an adjustable opening, mounted in the said air injection line downstream of the flow meter, a control signal for adjusting the opening of the said valve, the said control signal being the resultant of the signals generated by the prediction and correction computers.
The analysers, which are mounted in the line for injecting the gas to be treated into the oxidation reactor and on the output line of the said reactor, respectively, may be, for example, gas chromatography analytical units (U.S. Pat. No. 3,026,184 and FR-A-2,118,365), differential spectrometry analytical units (FR-A-2,420,754) or infrared absorption analytical units, after selective transformation of the H2S into SO2. 
The analysers of the aforementioned types, used for measuring the H2S content in gases containing this compound, do not always deliver continuous signals or do not always provide adequate sensitivity or adequate reliability, nor satisfactory operating simplicity.
The present invention provides an analyser for continuously measuring the H2S content of a gas containing it, which has a high sensitivity and the response of which shows no significant drift over time.
The analyser according to the invention, for continuously measuring the H2S content of a gas sample containing it, is characterized in that it comprises:
a dry-operating module for drying the gas sample, comprising an inlet, connected to a nozzle for taking and injecting the said sample, and an outlet for the dried sample;
a compressor module having a suction port, connected via a line to the outlet of the drying module, and a discharge port extended by a flow line for the compressed sample, the said line being equipped with an indicating and/or regulating primary flow meter;
a system for diluting the compressed sample, comprising an air intake line, which is mounted as a branch off the flow line for the compressed sample, downstream of the primary flow meter, and which is equipped with a regulating secondary flow meter adjusting the degree of opening of a valve having an adjustable opening, mounted in the air intake line downstream of the secondary flow meter, and a regulating module connected to each of the primary and secondary flow meters and slaving the secondary flow meter to the primary flow meter; and
an electrochemical sensor for measuring H2S, which is mounted in the flow line for the compressed sample, downstream of the air intake line, and delivers a signal proportional to the concentration of H2S in the said sample.
Advantageously, the nozzle for taking and injecting the gas sample, connected to the inlet of the dry-operating module for drying the gas sample, may be provided, at its remotest end from the said inlet, with a primary filter. Optionally, a finer filter may be provided at the other end of the said nozzle, located on the same side as the said module. If required, this nozzle may be surrounded by a jacket equipped with means for maintaining the temperature, for example by electrical heating or by the circulation of a heat-transfer fluid.
The dry-operating module for drying the gas sample may consist, in particular, of a dryer comprising permeation membranes such as the xe2x80x9cSECxe2x80x9d dryer sold by Environnement SA.
The compressor module may be chosen from various miniaturized compressors having the required performance. Particularly suitable are diaphragm compressors.
The indicating and/or regulating primary flow meter mounted in the flow line for the compressed sample, as well as the regulating secondary flow meter mounted in the air intake line are, in particular, mass flow meters. In this case, the regulating module which is associated with them is a mass-regulating module.
The electrochemical sensor for measuring the H2S concentration is of the electrochemical transducer type for measuring the partial pressure of the compound measured. This sensor comprises a measurement cell, which contains a liquid electrolyte, in which a measurement electrode, a comparison electrode and a reference electrode are immersed, and which is separated, by a membrane, from the flow space for the gas sample on which the measurement is made. A constant electrical voltage is maintained between the measurement electrode and the reference electrode. The gas sample containing the compound to be measured, in the present case H2S, diffuses through the membrane into the liquid electrolyte. The aforementioned electrical voltage, the electrolyte and the material of the electrodes are chosen so that the compound, the concentration of which is to be determined, is transformed electrochemically at the measurement electrode and so that an electric current of intensity proportional to the concentration of the said compound passes through the measurement cell. An electrochemical reaction occurs at the same time, at the comparison electrode, with the oxygen of the dilution air. Such a sensor delivers an electrical signal of intensity proportional to the concentration of the compound to be measured, in this case H2S, present in the flowing gas sample in contact with the sensor. As an example of an electrochemical sensor that can be used in the analyser according to the invention, mention may be made of the sensor sold by Drager under the name xe2x80x9cPolytron H2Sxe2x80x9d.
The analyser according to the invention, which allows the concentration of H2S contained, in relatively large amount, in a gas to be determined, is most particularly usable as an H2S analyser in an air-flow-regulating device with which a reactor for oxidizing H2S to sulphur is equipped. More especially, the analyser according to the invention can be used to form the analyser of a feedback unit and even the analyser of a prediction unit of the regulating device having the structure defined above, in order to regulate the flow rate of air injected into a reactor for oxidizing H2S to sulphur.
The invention will be more clearly understood on reading the description given below of one of its embodiments, made with reference to the appended drawings in which: